The invention relates to calipers or caliper instruments (also sometimes referred to as dividers) of the type utilized by persons who work in dark environments, for example paramedics needing to make measurements on a medical chart plotted by a heart monitoring instrument under “field conditions”, navigators, aviators, and other persons who must measure the distance between two points of a chart plotted by an instrument, a map, or other document under very low ambient illumination conditions. It is important that the “night vision” of such persons not be temporarily disrupted by the presence of unnecessarily bright “white” light on the document (or other surface) to allow measurement of the distance between two points on the document. A person whose eyes are open for approximately 15 to 20 minutes under dark conditions or very low ambient illumination conditions acquires what is known as “night vision”, wherein the person's eyes become much more sensitive to low levels of ambient illumination. If the eyes of a person who has acquired night vision are suddenly subjected to relatively bright light, especially white light, the person is subjected to what is referred to as “night blindness”, wherein the acquired night vision is lost. The person then must reacquire his/her night vision by remaining in no-light or very low-light conditions for another 15 to 20 minutes, during which he or she may be unable to recognize critically important things or occurrences in the low ambient light conditions in which the person is working.
Although a need for solving this problem has existed for long time, no practical means for solving the problem, such as a compact, reasonably inexpensive illuminated caliper instrument, has been provided.
U.S. Pat. No. 5,115,571, issued May 26, 1992, by present inventor Robert A. Mackin, discloses a caliper instrument including a gear mechanism wherein the upper ends of the caliper legs are interlocked by gear teeth so that the caliper legs of the symmetrically in opposite directions about a bisection axis.